Please check to see if it can be removed by clicking on the Firefox menu and clicking on "+Customize". If not please consider a clean reinstall of Firefox:

Certain Firefox problems can be solved by performing a Clean reinstall. This means you remove Firefox program files and then reinstall Firefox. Please follow these steps:

Note: You might want to print these steps or view them in another browser.

1. Download the latest Desktop version of Firefox from mozilla.org (or choose the download for your operating system and language from this page) and save the setup file to your computer.
2. After the download finishes, close all Firefox windows (or open the Firefox menu and click the close button ).
3. Delete the Firefox installation folder, which is located in one of these locations, by default:
   • Windows:
     • C:\Program Files\Mozilla Firefox
     • C:\Program Files (x86)\Mozilla Firefox
   • Mac: Delete Firefox from the Applications folder.
   • Linux: If you installed Firefox with the distro-based package manager, you should use the same way to uninstall it - see Install Firefox on Linux. If you downloaded and installed the binary package from the Firefox download page, simply remove the folder firefox in your home directory.
4. Now, go ahead and reinstall Firefox:
   1. Double-click the downloaded installation file and go through the steps of the installation wizard.
   2. Once the wizard is finished, choose to directly open Firefox after clicking the Finish button.

More information about reinstalling Firefox can be found here.

WARNING: Do not use a third party uninstaller as part of this process. Doing so could permanently delete your Firefox profile data, including but not limited to, extensions, cache, cookies, bookmarks, personal settings and saved passwords. These cannot be easily recovered unless they have been backed up to an external device! See Back up and restore information in Firefox profiles.

Please report back to say if this helped you!

Thank you.

If you haven't checked your Add-ons page in a while, also check there for unknown, suspicious, or unnecessary items. Either:

• Command+Shift+a
• "3-bar" menu button (or Tools menu) > Add-ons

In the left column, click Plugins. On the right side, if there are any you don't use or don't recognize, set them to "Never Activate".

In the left column, click Extensions. On the right side, if in doubt, disable.

Often a link will appear above at least one disabled extension to restart Firefox. You can complete your work on the tab and click one of the links as the last step.

Does that eliminate the mystery item? If so, please "name names" so others can find this thread.

"Blur" is the new combination of "Do Not Track Me", which works fine for me, and "Mask Me", the users of which seem very unhappy.

https://addons.mozilla.org/en-US/firefox/addon/donottrackplus/?src=search

The way the Add-ons site is designed, it is difficult to roll back and stick with an earlier version (if that's what you wanted to do). There is a pre-Blur version here that you could right-click>Save Link As to save locally:

https://addons.mozilla.org/firefox/addon/donottrackplus/versions/

Then if you drag and drop that onto any open Firefox tab, you should have the option to install it. But it might not pick up your existing DNT Me data (if the add-on has data)...